[OS X TeX] Illustrations appear as black boxes

[Adam Fenn]

I have a few large illustrations (2.2MB+) in my document which TeXshop 
displays as solid black boxes or, to be more accurate, negatives. 
Smaller illustrations display OK. If I open up the file in Adobe Reader 
the illustrations are displayed correctly and will print out. But if I 
leave the document open for any length of time the illustrations and 
fonts all disappear leaving blank pages. Is there anything I can do to 
remedy this?

TIA
Adam.